1. Technical Field
The present invention relates to a fixing device that is used in an image forming apparatus in which, for example, the electrophotographic system is employed, and more particularly to a fixing device that includes a rotatable belt member.
2. Related Art
In an image forming apparatus in which the electrophotographic system is used, such as a copier or a printer, image formation is performed in the following manner. First, the surface of a photosensitive member (photosensitive drum), which is formed into, for example, a drum-like shape, is uniformly charged by a charging device. The charged photosensitive drum is scan-exposed to light, which is controlled based on image information, and an electrostatic latent image is formed in the surface. The electrostatic latent image formed on the photosensitive drum is converted to a visible image (toner image) by a developing device. Then, the toner image is transported to a transfer station in accordance with rotation of the photosensitive drum and electrostatically transferred onto a recording sheet serving as a recording medium. Thereafter, a fixing process is applied on the toner image carried on the recording sheet, by a fixing device, and then an image is completed.
In a fixing device that is used in such an image forming apparatus, usually, a configuration that is called the two-roll system is widely used. A fixing device of the two-roll system is configured so that a fixing roll and a pressing roll are in press contact with each other. The fixing roll is formed by stacking a heat-resistant elastic layer and a separation layer on the surface of a cylindrical metal core, in which a heating source (heater) is placed. The pressing roll is formed by stacking a heat-resistant elastic layer and a separation layer formed by a heat-resistant resin film or a heat-resistant rubber film on a metal core. A recording sheet carrying an unfixed toner image is passed through a press contact region (nip portion) between the fixing roll and the pressing roll to heat and pressurize the unfixed toner image, thereby fixing the toner image.
In the field of an image forming apparatus, recently, enhancement of productivity and colorization are rapidly advancing, and an image forming apparatus having a double-sided printing mechanism is widely used. Also in a fixing device to be mounted on an image forming apparatus, therefore, a countermeasure against an increased speed is required to be further advanced.
However, a conventional fixing device of the two-roll system has a problem in that a sufficient fixing process is hardly performed on a large number of recording sheets that are consecutively fed at high speed. Namely, in a fixing device of the two-roll system, a metal core constituting a fixing roll, and an elastic layer made of silicone rubber or the like covering the metal core function as a thermal resistor. In a fixing device of the two-roll system, therefore, it is structurally difficult that a heater disposed inside a fixing roll supplies adaptively and sufficiently a quantity of heat corresponding to that which is captured by the recording sheet from the surface of the fixing roll.
As a result, when recording sheets are consecutively fed at high speed to a fixing device of the two-roll system, there arises a disadvantage that the surface temperature of a fixing roll is gradually lowered, and the fixing performance is progressively impaired. In a starting period of an image forming apparatus, so-called “temperature droop phenomenon” in which the surface temperature of a fixing roll temporarily drops easily occurs. When a thick sheet or the like having a large heat capacity is used as a recording sheet, particularly, a large quantity of heat is captured from the surface of the fixing roll, and hence the reduction of the fixing performance and the temperature droop are increased, with the result that deterioration of the image quality due to a fixing failure occurs.
A technique for realizing a fixing device that solves the above-mentioned problems caused when a fixing device of the two-roll system is used and that deals with the speeding up of an image forming apparatus has been developed. For example, there is a technique relating to a fixing device in which a heating member for heating a recording sheet is configured by a film-like belt member (fixing belt) mounted on and tensioned by plural support rolls.
Recording sheets have different heat capacities depending on the property such as the thickness. Therefore, the quantity of heat that is captured from the fixing belt by a recording sheet during the fixing process is different depending on, for example, the thickness. When compared with the case where sheets are consecutively passed at a process speed (sheet passing speed) and a set temperature that are set with reference to a thin recording sheet (thin sheet), a thick recording sheet (thick sheet) has a heat capacity larger than that of a thin sheet, and the quantity of heat that is used for fixing a toner is reduced. Therefore, there may arise a disadvantage that a fixing failure easily occurs.
A configuration where the process speed is switched over in accordance with the thickness of a recording sheet may be contemplated. Namely, in the case of a thick sheet, the process speed is set to be slower than the case of a thin sheet, so that the quantity of heat captured from the fixing roll by the recording sheet is suppressed, thereby preventing the temperature of the fixing roll from being lowered.
In the configuration where the process speed is switched over in cases of a thin sheet and a thick sheet in this way, however, the operating efficiency of the case of a thick sheet is lower than that of the case of a thin sheet.
By contrast, a configuration where the process speed is maintained constant and the set temperatures of the fixing belt and fixing roll are switched over in accordance with the thickness of a recording sheet (the temperatures are made higher in the case of a thick sheet) may be contemplated.
When it is configured so that the set temperatures are switched over in accordance with the sheet thickness, however, there is a problem that the switching process requires a time period. In the case where a thin sheet is switched to a thick sheet, a temperature rise time period that is considerably long elapses before the fixing roll-reaches a higher set temperature, because it is difficult to rapidly heat the fixing roll having a large heat capacity. By contrast, in the case where a thick sheet is switched to a thin sheet, a temperature fall time that is considerably long elapses before the fixing roll reaches a lower set temperature. Consequently, there is a problem that the operating efficiency is inevitably lowered.